1. Technical Field
The present invention relates in general to an improved apparatus for reducing the skin friction drag of an aerodynamic or hydrodynamic surface, and in particular to an improved riblet design for reducing the skin friction drag coefficient of aerodynamic or hydrodynamic surfaces.
2. Description of the Prior Art
It is relatively well known that the aerodynamic drag of a surface may be reduced by applying a microscopic xe2x80x9ctexturexe2x80x9d to the otherwise smooth surface. Although the exact fluid dynamic mechanism at work in this drag reduction is not well understood, it is speculated that the reduction relates to controlling the turbulent vortices in the boundary layer adjacent to the surface. The microscopic texture reduces the skin friction drag of solids moving through fluids (e.g., aircraft, ships, cars, etc.), and of fluids moving along solids (e.g., pipe flow, etc.). Although the practical use of such texturing has been very limited, one well known exception was the application of a texture to the racing yacht, Stars and Stripes.
The geometric form for the microscopic, friction-reducing texture is known as xe2x80x9criblets.xe2x80x9d As shown in FIG. 1, riblets 11 are typically an integrated series of groove-like peaks 13 and valleys 15 with V-shaped cross-sections. Riblets 11 always extend along the aerodynamic surface 17 in the direction of fluid flow 19. The height of the riblets and the spacing between the riblets are usually uniform and on the order of 0.001 to 0.01 inches for most applications. Dimensionless units, sometimes referred to as wall units, are conventionally utilized in describing fluid flows of this type. The wall unit h+ is the non-dimensional distance away from the wetted surface or more precisely in the direction normal to the surface, extending into the fluid. Thus h+ is a non-dimensional measurement of the height of the riblets. The wall unit s+ is the non-dimensional distance tangent to the local surface and perpendicular to the flow direction, thus the non-dimensional distance between the riblets. In the prior art riblets, h+ and s+ are in the range between 10 and 20. Previous riblet designs consisted of an adhesive film applied to a smooth solid surface. However, with advanced manufacturing techniques, the same shapes may be directly formed and integrated into the structure of the aerodynamic surface.
The interaction of riblets with the structure of the turbulent boundary layer of the fluid reduces the skin friction drag coefficient (Cdf) of the surface by approximately 6% compared to an identical smooth surface without riblets. This reduction occurs despite the significant increase in xe2x80x9cwetted areaxe2x80x9d (the surface area exposed to the fluid stream) of a riblet-covered surface over a smooth surface. In attempts to further reduce the Cdf, modifications to conventional V-shaped riblets have been proposed. Examples include rounding of the peaks 21 and/or valleys 23 (FIG. 2), as well as even smaller V-shaped notches 31 in the sides of the larger V-shaped riblets 33 (FIG. 3). In summary, all of the work has been with riblets having a constant geometry or cross-section in the streamwise direction. An improved riblet design that decreases skin friction drag with less concomitant increase in wetted area than conventional riblets would be desired.
A series of parallel riblets extend from a smooth, aerodynamic surface for reducing the skin friction drag of the surface an airstream flows around it. The riblets extend longitudinally along the surface and have a triangular cross-section in the transverse direction. The apex of the cross-section defines a continuous, undulated ridge with peaks and valleys. Measured from the surface, the peaks have a greater height than the valleys. The interaction of the riblets with the structure of the turbulent boundary layer of the airstream reduces the skin friction drag coefficient of the surface by approximately 8% to 20% over an identical smooth surface without the riblets. The reduction is better than the 6% reported for conventional riblets because the wetted area is increased less with this invention than with conventional riblets.